Switch

ABSTRACT

A switch comprises voltage applying means for providing direct current potentials to first to third beams arranged with a spacing slightly distant one from another, and electrodes for inputting/outputting signals to/from the beams. By controlling the direct current potential provided to the beam, an electrostatic force is caused to thereby change the beam positions and change a capacitance between the beams. By causing an electrostatic force between the first and second beams and moving the both beams, the first and second beams can be electrically coupled together at high speed. Also, an electrostatic force is caused on the third beam arranged facing to the first and second beams, to previously place it close to the first and second beams. When the electrostatic force is released from between the first and second beams, the second beam moves toward the third beam thereby releasing the first and second beams of an electric coupling.

FIELD OF THE INVENTION

[0001] This invention relates to a switch, for use on an electriccircuit, having an electrode to be mechanically moved by an externallyapplied force, to thereby pass or cut off the signal.

BACKGROUND OF THE INVENTION

[0002] Conventionally there is known, as a switch for use on an electriccircuit, a switch using an air bridge described in U.S. Pat. No.6,218,911. In this structure, a movable air bridge is arranged between apair of electrodes formed on a substrate. In case an electrostatic forceis given between the electrode and the movable air bridge, the airbridge horizontally moves toward the electrode into a contact with oneelectrode but isolated from the other electrode. Accordingly, in case asignal is inputted to the air bridge, the air bridge is electricallyconnected with the one electrode, allowing a signal to pass. However,the signal is cut off at the other electrode, thus enabling switchoperation.

[0003] Meanwhile, a micro-electromechanical RF switch is known which isdescribed in U.S. Pat. No. 6,307,452. The micro-electromechanical RFswitch has a plurality of folded spring suspension devices on asubstrate, on which a micro-platform is suspended. Beneath themicro-platform, a signal line is formed. When a direct current potentialis applied between the signal line and the micro-platform, anelectrostatic force is caused to attract the micro-platform toward thesignal line, thus effecting switch-on.

[0004] However, in the structure of U.S. Pat. No. 6,218,911, in the caseof driving the air bridge on an electrostatic force, realizing greatersignal isolation requires to increase the spacing between the electrodeand the air bridge. However, because electrostatic force is proportionalto a negative square of distance, electrostatic force decreases andmakes it impossible for response time to attain a desired value.Meanwhile, there is an approach to increase the application voltage inorder to compensate for the decrease of electrostatic force. However,application voltage increase is not preferred for the radiocommunication device requiring low power consumption and low drivevoltage.

[0005] Meanwhile, because the air bridge is of a straight-beamstructure, tensile stress if exists within the beam increases therigidity against electrostatic force just like a strongly stretchedcord, raising a pull-in voltage (pull-in voltage due to electrostaticforce). Furthermore, at an elevated temperature, beam internal stressturns into compression, possibly causing buckling. Namely, unless theresidual stress resulting from a manufacture process or environmentaltemperature upon switch operation can be controlled constant, stableswitch operation characteristic cannot be guaranteed.

[0006] On the other hand, the micro-platform structure in U.S. Pat. No.6,307,452 is divided with a region for coupling to a signal line and afolded spring-suspension structure part (flexure) for relaxing stress.Namely, an additional structure is provided to relax internal stress. Asapparent from Newton's laws of motion, in the case of applying the sameforce to a structure having a mass m, the acceleration occurring on thestructure is greater as the mass m is smaller. For this reason, theabove structure involves the problem that, because of addition of theflexure, the mass m is increased to make it impossible to increase theresponse speed. Meanwhile, as the flexure is softer, the platform isrelaxed in binding at its supports. Consequently, in case there exists astress gradient in a direction of film thickness, the platform warps updue to stress release and separates off the substrate. Unless the stressgradient value cannot be accurately reproduced in the beam manufactureprocess, the degree of warpage varies, making it impossible to suppressthe variation in capacitance reduction between a platform and a signalline and the variation in pull-in voltage increase. Meanwhile, themanufacture with using a semiconductor process makes a beam and aflexure structure into the same material of conductors. In a radiofrequency circuit, the flexure part thereof has an non-negligibleimpedance.

[0007] Meanwhile, where the environmental temperature changes, thermalstress takes place due to a difference of thermal expansion coefficientbetween the base material and the beam material. Although the thermalstress is different in occurrence cause from the foregoing residualstress encountered in manufacture process, it triggers a phenomenon ofthe similar “strain in the beam due to stress release”. Accordingly, itmust be taken into account of an effect upon capacitance or pull-involtage.

SUMMARY OF THE INVENTION

[0008] The present invention has been made in view of the foregoingpoints, and it is an object thereof to provide a switch capable ofrealizing to shorten response time and reduce application voltage.

[0009] Also, another object is to provide a switch capable of realizinga switch free of a variation in pull-in voltage increase.

[0010] Also, another object is to provide a switch capable ofsuppressing the change of switch characteristic due to a beam internalstress change.

[0011] A switch of the present invention is structured by first, secondand third beams arranged with spacing slightly distant one from another,voltage applying means for independently providing the beams with directcurrent potentials to apply an electrostatic force to the beam, andelectrodes provided on the beams and to input/output an alternatingcurrent signal to/from the beam whereby the beams are changed inposition by the electrostatic force and changed in the capacitancebetween the beams.

[0012] According to this structure, an electrostatic force is causedbetween the first and second beams to thereby move both of the first andsecond beams so that the beams can be coupled together at high speed andput off at high speed. By causing an electrostatic force on the thirdbeam arranged facing to the second beam and previously placing it closeto the first and second beams, a strong electrostatic force can beapplied between the second and third beams, enabling to make a responseat higher speed.

[0013] Also, in the invention, by providing the beams with the same formof bending, it is possible to relax a pull-in voltage change against abeam internal stress change and also a beam-to-beam capacitance changedue to beam strain.

[0014] This makes it possible to structure a ultra-small-sized variablecapacitive switch which is to be driven at high speed on low voltage andreduced in the characteristic change due to residual stress or thermalexpansion, by the use of a semiconductor thin-film process.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a perspective view showing a schematic structure of aswitch according to embodiment 1 of the present invention;

[0016]FIG. 2A is a switch connection circuit diagram according toembodiment 1 of the invention while FIG. 2B is an equivalent circuitdiagram of the same switch;

[0017] FIGS. 3A-3F are a concept view explaining the operation of theswitch of embodiment 1 of the invention;

[0018] FIGS. 4A-4F are a sectional view showing one example of a processto manufacture a switch of embodiment 1 of the invention;

[0019]FIG. 5 is an essential-part sectional view of a switch accordingto embodiment 2 of the invention;

[0020]FIG. 6 is an equivalent circuit diagram of a switch according toembodiment 3 of the invention;

[0021]FIG. 7 is a plan view showing a schematic structure of a switchaccording to embodiment 3 of the invention;

[0022]FIG. 8A is a perspective view showing a schematic structure of aswitch according to embodiment 4 of the invention while FIG. 8B is aplan view of the same switch;

[0023]FIG. 9 is a characteristic diagram showing a relationship betweena beam internal stress and a pull-in voltage of a switch according toembodiment 4 of the invention;

[0024]FIG. 10 is a characteristic diagram showing a relationship betweena beam internal stress and a beam-to-beam capacitance of the switchaccording to embodiment 4 of the invention;

[0025]FIG. 11 is a sectional view explaining one example of amanufacturing method for a switch of embodiment 4 of the invention;

[0026]FIG. 12 is a characteristic diagram showing a relationship betweena beam internal stress and a beam primary resonant frequency of theswitch according to embodiment 4 of the invention;

[0027]FIGS. 13A and 13B are a concept view explaining a structure andoperation of a switch according to embodiment 5 of the invention;

[0028]FIG. 14 is a characteristic diagram showing a relationship betweena movable material application voltage and an internal stress of theswitch according to embodiment 5 of the invention; and

[0029]FIG. 15 is a concept view explaining a control method for a switchaccording to embodiment 5 of the invention.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

[0030] Exemplary embodiments of the present invention are demonstratedhereinafter with reference to the accompanying drawings.

[0031] The present invention has a gist to realize, in a switch havingthree beams to be changed in relative positions so that the capacitancecan be changed between the beams, to provide electric coupling anddecoupling, a structure that high-speed switching and low direct-currentcontrol is made possible by making the beams all movable.

[0032] Meanwhile, the present structure aims at relaxing the pull-involtage change against a beam internal stress, to relax also thebeam-to-beam capacitance change resulting from beam strain, byconstructing the beams forming the switch by a flexure structure.

1. First Exemplary Embodiment

[0033] With reference to FIGS. 1 to 3, explained is embodiment 1 of theinvention. FIG. 1 depicts a schematic structural view of a switchaccording to embodiment 1. A first beam 1, second beam 2, third beam 3is formed of such a shape and material as transferring an electricsignal with no loss, having an insulation film with approximately 10 nmon a surface thereof. The beam 1, 2, 3 is formed, for example, of ametal, such as Al, Au, Cu or an alloy, having a shape in aboth-ends-supported beam structure having a thickness 2 μm, a width 2 μmand a length 200 μm and supported at both ends. These are arrangedparallel at such a spacing, e.g. of 0.6 μm, to satisfy a givenisolation. The beam 1, 2, 3 is not necessarily a both-ends-supportedbeam structure but may be a cantilever form. Meanwhile, the beam 1, 2, 3has a beam spring constant to be varied by changing the shape.Incidentally, the beam 1, 2, 3 is based on a structure and process toreduce its internal stress to a possible less extent, the detail ofwhich will be referred later. The beam 1 has both ends connected withelectrodes 4, 7, the beam 2 with electrodes 5, 8, and the beam 3 withelectrodes 6, 9.

[0034] In order for easy explaining, explanation is made on an examplethat the electrode 5 is taken as an input terminal to be applied by aninput signal, the electrode 7 is taken as an output terminal connectedto an antenna end, and the electrode 9 is terminated at 50 Ω. FIG. 2Ashows a connection circuit while FIG. 2B shows an equivalent circuitthereof.

[0035] In following explanation, “switched on” means a state of placingthe electrode 5 and the electrode 7 into contact in FIG. 2A, and“switched off” means a state of isolating the electrode 5 and theelectrode 7 and placing the electrode 5 and the electrode 9 intocontact. According to the circuits shown in FIGS. 2A and 2B, noreflected wave is generated when switched off, because the circuits areterminated at 50 Ω. In addition, the impedance of the capacity c₁becomes large and the impedance of the capacity c₂ becomes small whenswitched off, so that the signal from the input terminal is groundedthrough the capacity C₂ and the 50 Ω resistor. As a result, theisolation becomes large between the electrode 7 and the electrode 5. Inthis case, it is preferable to insert capacitors between the inputsignal source and input terminal electrode 5, and antenna and outputterminal electrode 7. The 50 Ω resistor may be omittable for enhancingisolation between the electrode 7 and the electrode 5.

[0036] In this configuration, no reflection wavestake place as viewedfrom the input terminal. Furthermore, when the switch is OFF, isolationis to be taken great at between the electrode 7, as an antenna end, andthe electrode 5, as an input terminal. In this case, capacitances may bedisposed at between the electrode 5, as an input signal source and theinput terminal, or input terminal, and between the electrode 7, oroutput terminal, and the antenna end, as required.

[0037] Incidentally, connecting the electrode 9 to another outputterminal instead of termination, it is possible to realize adistribution switch having 1 input and 2 outputs. Otherwise, in case theelectrode 5 is taken as an output terminal and the electrodes 7 and 9 asinput terminals from the antenna, a selector switch can be made having 1output versus 2 antenna inputs.

[0038] Now, switch operation is explained with using FIGS. 3A to 3F.FIG. 3A shows a state that no voltages are applied to the electrodes 4-9of the FIG. 1 switch. In order to couple a signal from the inputterminal to the antenna end, in FIG. 3B, the direct current potential bya control voltage source 10 connected to the electrode 4 is set at apredetermined response time High. Similarly, the direct currentpotential of a control voltage source 11 connected to the terminal 5 andthe direct current potential by a control voltage source 12 connected tothe terminal 6 are set at a predetermined response time Low. Due tothis, an electrostatic force is caused between the beam 1 and the beam2. The beam 1 and the beam 2 are attracted into contact with each other.

[0039] At this time, in case the beam 1 and the beam 2 are in the sameform with a same spring constant and mass, the beam 1 and the beam 2 areplaced in contact at a halfway point. In this case, as compared to thecase that either one of the beams 1, 2 is provided as a fixed electrode,because the distance change amount between the beams 1, 2 under the sameelectromagnetic force is twice the amount. Response is possible athigher speed. With the same response time, control is possible at lowervoltage. For example, in case the electrode 4 is given a direct currentpotential 7.25 V, response time can be 5 μS or less. However, in thecase it is movable only at one end, response time is 7.4 μS, i.e. aresponse time is longer approximately 1.5 times. In this case, in orderto reduce a response time down to 5 μS, application voltage must be at10.3 V.

[0040] When the beam 1 and the beam 2 come into contact, the alternatingcurrent signal inputted at the electrode 5, or input terminal, istransferred from the beam 2 to the beam 1 by a capacitive couplingthrough the insulation film provided on the surface of the beam 1, 2,thus being outputted onto the electrode 7, or output terminal.

[0041] In the state of FIG. 3B, in case the direct current potential bythe control voltage source 12 connected to the electrode 6 of the beam 3is rendered High, an electrostatic force occurs at between the beam 3and the beam 2. Thus, the beam 3 moves in a direction toward the beam 2,as shown in FIG. 3C. At this time, the beams 1, 2 also move in adirection toward the beam 3. However, because the beams 1, 2, coupledtwo in the number, they are great in equivalent spring constant, movingamount is small as compared to that of the beam 3. However, it is notedthat the direct current potential to be applied to the electrode 6 is ata voltage not to pull-in the beam 3 or smaller. Under the foregoingcondition, the pull-in voltage is approximately 6.7 V. If a voltage lessthan that is applied, the beam 3 has a maximum displacing amount ofnearly 0.15 μm, and the beams 2, 3 have a maximum gap of 0.75 μm.Because electrostatic force is inversely proportional to a square ofdistance, the electrostatic force caused between the beams 3 and 2 is1.4 times as great as that of the case the beam 3 is not moved.

[0042] Incidentally, instead of applying a direct current potential tothe electrode 6 in the state of FIG. 3B, the direct current potentialson the electrodes 4 and 5 may be instantaneously reversed to each other.By doing so, an electrostatic force can be caused between the beams 2and 3 without newly applying a direct current potential by the controlvoltage source 12. In this case, there is no possibility of causingpull-in because of a great gap at between beams 2 and 3.

[0043] Meanwhile, in a situation isolation is required high, in case thedirect current potential by the control voltage source 12 is kept in theLow state, the beam 3 is not to move. This can maintain the state thegap between the beams 2, 3 is kept great, making it possible to decreasethe electric coupling between the beams 2 and 3.

[0044] Now, explained is the operation the input signal is switched andoutputted, as antenna end, from the electrode 7 to the electrode 9. Inthe state of FIG. 3C, the direct current potential being applied to theelectrode 4 is turned from High to Low, an electrostatic force does notoccur at between the beams 1 and 2. Consequently, the beam 1 and beam 2is returned to its former position by its own spring force, as shown inFIG. 3D. At this time, because the beam 3 is previously deformed towardthe beam 2, the beam 2 is strongly, rapidly moved toward the beam 3 byan electrostatic force caused between the beams 2, 3 and placed intocontact with the beam 3, as shown in FIG. 3E. In the case the beam 3 isnot previously deflected toward the beam 2, the maximum gap is 0.9 μm.This requires a higher voltage to be applied in shortening the responsetime.

[0045] When the beam 2 and the beam 3 come into contact, the alternatingcurrent signal inputted at the electrode 5 is transferred from the beam2 to the beam 3 by a capacitive coupling through the insulation filmformed on the surface of the beams 2 and 3, thus being outputted ontothe electrode 9. By connecting the beams 2 and 3 when switched off, C2is short-circuited and c1 becomes hard to transmit signals in FIG. 2B,so that higher isolation is obtained between the electrode 7 and theelectrode 5.

[0046] When the beam 3 is not bended previously towards the beam 2,maximum gap between the beams 2 and 3 becomes about 0.9 μm, it isnecessary to supply high control voltage to the beam 2 for operating theswitch within desired short response time.

[0047] Incidentally, in the state of FIG. 3E, a direct current potentialis further applied to the electrode 4 similarly to the case of FIG. 3Cto thereby apply an electrostatic force at between the beams 1 and 2,the beam 1 deflects toward the beam 2 as in FIG. 3F, enabling to reducethe maximum gap.

[0048] By the switch operation as above, the beam 2 applied by a signalin ON and OFF states is always in contact with the other beam 1 or 3,i.e. in a latched state. Due to this, should a great power signal beinputted to the beam 2, the beam 2 unless being latched is possiblyattracted due to an electrostatic force of the signal itself by the beam1 or 3. However, because of always latched by the beam 1 or 3, the beam2 can be prevented from malfunctioning.

[0049] Although the above explained the case that the beams 1, 2, 3 areto move horizontally due to an electrostatic force, the beams 1, 2, 3may be arranged in a vertical direction and to be moved vertically.Meanwhile, electrostatic force is used in a driving force,electromagnetic force, piezoelectricity or heat may be used instead.Besides in air, the beams 1, 2, 3 may be operated in vacuum or in aninert gas.

[0050] Now, explained is one process example to manufacture a switch ofFIG. 1, with using a process sectional view of FIG. 4. In FIG. 4A, whena high resistive silicon substrate 41 is thermally oxidized, a siliconoxide film 42 is formed in a thickness of approximately 300 nm on thesubstrate 41. A silicon nitride film 43 is deposited over that with afilm thickness of 200 nm, by a lowpressure CVD process. Furthermore, asilicon oxide film 44 is deposited on that with a film thickness of 50nm, by a low pressure CVD process.

[0051] Then, in FIG. 4B, a sacrificial layer of photoresist isspin-coated with a film thickness of 2 μm over the silicon oxide film44. After exposure to light and development, baking is carried out on ahot plate at 140° C. for 10 minutes, thereby forming a sacrificial layer45.

[0052] Thereafter, as shown in FIG. 4C, an Al layer 46 is deposited witha film thickness of 2 μm over the entire substrate surface, bysputtering. Thus, a photoresist pattern 47 is formed leaving the resistin a predetermined area.

[0053] Next, as shown in FIG. 4D, the photoresist pattern 47 is used asa mask, to carry out dry etching on the Al layer 46 thereby forming abeam 48. Furthermore, an oxide plasma process is carried out to removethe photoresist pattern 47 and sacrificial layer 45. By the aboveprocess, formed is the beam 48 having a gap 49 to a surface of thesubstrate 41.

[0054] Furthermore, as shown in FIG. 4E, a silicon nitride film 50 isdeposited with a film thickness of 50 nm on the entire surface of beam48 and over the silicon oxide film 44 on the substrate surface, by aplasma CVD. Thereupon, a silicon nitride film 50 is formed over thesilicon oxide film 44 on the substrate surface and on the periphery ofthe beam 48.

[0055] Finally, as shown in FIG. 4F, etching back is made on the siliconnitride film 43 by a dry etching process having anisotropy, under thecondition of a selective ratio of a film thickness greater than theforegoing deposition film thickness, e.g. 100 nm, to the silicon oxidefilm 44. Etching is made not to have the silicon nitride film 50 on anupper surface but leave the silicon nitride film 50 at only a sidesurface, thus forming a beam 51.

[0056] Incidentally, although this embodiment used the high resistivesilicon substrate 41, a usual silicon substrate, compound semiconductorsubstrate or insulation-material substrate may be used alternatively.

[0057] Also, although a silicon oxide film 42, a silicon nitride film 43and a silicon oxide film 44 were formed as insulation films on the highresistive silicon substrate 41, these insulation films may be omittedlyformed where substrate resistance is sufficiently high. Meanwhile, onthe silicon substrate was formed an insulation film in a three-layeredstructure having a silicon oxide film 42, a silicon nitride film 43 anda silicon oxide film 44. However, in the case the silicon nitride film43 has a film thickness sufficiently greater as compared to a siliconnitride film deposited on the beam, i.e. a film thickness not to vanisheven through so-called an etch-back process, it is possible to omit theforming process for a silicon oxide film 44.

[0058] Incidentally, this embodiment used Al as a material for formingthe beam. Alternatively used may be another metal material, e.g. Mo. Ti,Au or Cu, a semiconductor material such as amorphous silicon introducedwith an impurity with concentration, or a polymer material havingconductivity. Furthermore, although sputtering was used as a filmforming process, forming may be by using a CVD technique, a platingtechnique or the like.

2. Second Exemplary Embodiment

[0059] Now a second embodiment is explained while referring to FIG. 5.This embodiment is basically the same in structure as the firstembodiment. However, a second beam 32 is formed smaller in thickness ascompared to the first beam 31 and third beam 33, e.g. the first andthird beams are formed 1.5 times greater in thickness than the secondbeam. In this embodiment, when the first beam 31 and the second beam 32come into contact, an electrostatic force 35 acts between the first beam31 and the third beam 33 in addition to an electrostatic force 34 actingbetween the first beam 31 and the second beam 32. With this structure,even unless a direct current potential is newly applied to the electrode6 after a contact between the first beam 31 and the second beam 32 aswas in the first embodiment, the third beam 33 is to move toward thesecond beam 32.

[0060] In such a case, in order for the first beam 31 to near toward thethird beam 33 to a possible close extent, the second beam 32 may have anincreased spring constant so that the first beam 31 and the second beam32 can go into contact not at a halfway point but a point closer to thesecond beam 32.

3. Third Exemplary Embodiment

[0061] Now a third embodiment is explained while referring to FIGS. 6and 7. This embodiment has a plurality (four in FIG. 6) of FIG. 2Aswitch circuits symmetrically about an antenna end 65, as shown in FIG.6. This can realize a one-input multi-output switch that can distributean input to one antenna into a plurality of outputs and multi-outputthem. The switch thus structured can be configured by arraying theswitches used in embodiment 1 and capacitively coupling those as shownin FIG. 7. Incidentally, FIG. 7 shows a case having two switch circuits.In FIG. 7, an electrode 71 is formed with a plurality of beams 74 in acomb form, having beams 75 between the beams 74. The beams 75 arerespectively coupled with electrodes 72. An electrode 73 is providedoppositely to the electrodes 72. The electrode 71 is connected with acontrol voltage source 76, the electrode 72 with a control voltagesource 77 and the electrode 73 is with control voltage source 78,respectively.

[0062] In case the direct current potential by the control voltagesource 76 connected to the electrode 71 is provided High while thedirect current potential by the control voltage source 77 connected tothe electrode 72 and the direct current potential by the control voltagesource 78 connected to the electrode 73 are provided Low, then acapacitive coupling 79 occurs at between the beam 74 and the beam 75thereby effecting switch operation.

[0063] In the case a quick response time is required on the embodiment 1switch, the moving beam must be small in mass. However, for theembodiment 3 switch for capacitive coupling, reducing a beam massresults in a reduction in the sectional area of capacitive coupling, todecrease a coupling degree and increase a passing loss. For this reason,in order to compatibly provide two reciprocal characteristics, i.e.response time and passing loss, the individual beams are made small toreduce the response time. By arraying such beams, the coupling degree isincreased on the switch overall thereby satisfying the twocharacteristics of response time and passing loss. For example, providedthat the individual beam is given a form having a width 2.5 μm by athickness 2.5 μm by a length 380 μm, 5 sets of switches in parallelarrangement provides a preferred passing characteristic at analternating current signal frequency of 5 GHz.

[0064] This embodiment has a frequency characteristic because ofcapacitive coupling. Provided that the switch capacitance on aseries-connection side shown in the equivalent circuit of FIG. 2B is C₁and the capacitance on a grounding side is C₂, impedance Z is to beexpressed as Equation 1. C₁ and C₂ use the switch having basically thesame configuration. The relationship between C₁ and C₂ is expressed asEquation 2. α represents a change ratio of capacitance, which is a ratioof a beam-to-beam gap and an insulation film thickness as it is.$\begin{matrix}{Z = {\frac{\omega \quad C_{1}}{1 - {\omega^{2}C_{1}C_{1}}}}} & {{EQ}\quad 1}\end{matrix}$

C₂=α C₁   EQ 2

[0065] In case α is taken great, drive voltage is increased to increaseresponse time. Accordingly, it cannot be taken so great. For example, inthe case an insulation film is 10 nm and a gap is 0.6 μm, α is given 60.

[0066] In order to secure isolation, the condition that impedance takesa maximum is shown by Equation 3. Provided that α is 60 and applicationfrequency is 5 GHz, C₁ is 4.2 pF. If this is replaced into a form ofbeam, it is satisfactory to use five sets of beams each having athickness 2.5 μm by a width 2.5 μm by a length 380 μm. $\begin{matrix}{C_{1} = \sqrt{\frac{1}{\alpha \quad \omega^{2}}}} & {{EQ}\quad 3}\end{matrix}$

[0067] Meanwhile, when handling a signal having a frequency of 1 GHz, incase the frequency is one-fifth and hence the number of applicationbeams is given 5 times, i.e. 25 sets, a characteristic is obtainedequivalent to 5 GHz, thus enabling to realize a switch not having afrequency characteristic.

[0068] According to this embodiment, a switch having a desired impedanceor capacitance can be realized by arranging a plurality of switches inparallel.

4. Fourth Exemplary Embodiment

[0069] Now, embodiment 4 of the invention is explained while referringto FIGS. 8 to 12. FIG. 8A is a perspective view of a switch concernedwith embodiment 4 of the invention while FIG. 8B is a plan view thereof.A first beam 81, second beam 82 and third beam 83 is both-ends-supportedbeam whose both ends are fixed on a substrate (not shown) by anchorparts 84, 85. These are in a thickness t1=t2=t3=2 μm by a widthW1=W2=W3=2 μm by a length L=500 μm. The beam uses, as a material, Alhaving a Young's Modulus of 77 GPa. The beams 81, 82, 83 are arrangedparallel at an interval of g=0.6 μm. Insulation layers havingapproximately 0.01 μm are formed on the opposed side surfaces ofadjacent beams. This is sufficiently small as compared to the width ofbeam, having a less effect upon the mechanical characteristics of thebeam. Incidentally, the insulation film may be formed either one of orboth of the opposed side surfaces.

[0070] As shown in FIG. 8B, the beams 81, 82, 83 are curved in an S-formas viewed at the above of the switch. The S-form is expressed by oneperiod of a sinusoidal function of Equation 4, for example.$\begin{matrix}{y = {\Delta \quad y\quad {\sin \left( {2\pi \frac{x}{L}} \right)}}} & {{EQ}\quad 4}\end{matrix}$

[0071] Note that, in FIG. 8B, flexure is depicted with exaggeration inorder for easy understanding. On the beam, there exist an internalstress Sx in the x direction and internal stress Sy in the y directionevenly without relying upon x, y, z position. These are isotropicinternal stresses, i.e. Sx=Sy=S. The beam, to be manufactured by using asemiconductor process, is formed on a sacrificial layer. In this case,although there exists an internal stress S, the stress S removed of thesacrificial layer takes a somewhat freed value.

[0072] In the structure of FIGS. 8A and 8B, when the first beam 81 andthe second beam 82 are deflected by giving a potential difference tobetween these, internal stress S and pull-in voltage have a relationshipas shown in FIG. 9. Compared is the magnitude of flexure, i.e. the casesthe Δy value in Equation 4 is 2, 4 and 6 μm. Meanwhile, shown togetheris a case of a straight beam structure having Δy=0, i.e. having noflexure. However, because buckling occurs under the application of acompression stress, stress S is shown within a plus range, i.e. onlyvalues of upon tensile stress. In this manner, the increase of pull-involtage due to an increase of internal stress S can be suppressed bymerely giving a flexure. This provides a greater suppressing effect asthe magnitude of flexure, i.e. Δy value is increased.

[0073] Now, explained is the case of a flexure in an arch form, in orderto verify the effect of S-form. The arch-formed flexure was approximatedby a half period of a sinusoidal function of Equation 5. Therelationship of an internal stress S and a pull-in voltage at Δy=4 μm istogether shown in FIG. 9. $\begin{matrix}{y = {\Delta \quad y\quad {\sin \left( {\pi \frac{x}{L}} \right)}}} & {{EQ}\quad 5}\end{matrix}$

[0074] Apparently, an arch form at S=0-30 MPa is greater in pull-involtage than an S-form having Δy=2 μm, wherein they soon go near in aregion of 30 MPa or greater. In a range of S=0-10 MPa, it has a greaterpull-in voltage rather than the straight beam. Nevertheless, becausepull-in voltage is nearly constant at around S=20±10 MPa, the variationin pull-in voltage can be reduced if the variation in residual stresscan be suppressed within that range.

[0075] Next, by providing the same flexure in the adjacent beams, it ispossible to suppress the capacitance change at between the adjacentbeams against a deformation of the beam due to internal stress. FIG. 10represents a relationship between an internal stress S and a capacitanceat a potential difference of 0V between the adjacent beams. In caseplotting is made on three S-forms (Δy=2 μm, Δy=4 μm, and Δy=6 μm)different in flexure degree and an arch-form (Δy=4 μm), it can be seenthat the four are overlapped one with another as the curve-w.Accordingly, capacitance is kept nearly constant both on the arch formand S-form without undergoing the effect of internal stress. Namely,even where the beam internal stress is changed by a variation inmanufacture process or by a thermal expansion due to a surroundingtemperature change, electric characteristic variation can be suppressedas a capacitive coupling type switch.

[0076] Incidentally, the beams 81, 82, 83 are of the same flexure formand hence the same mechanical springiness. In case a potentialdifference is given, for example, between the beams 81, 82, the bothdisplace the same amount into a contact at a half point in the gapbetween the both. For example, in order to near this contact pointtoward the beam 81, it is satisfactory to increase the rigidity of thebeam 81. The first method is to increase the width W of the beam 81.There is shown, in FIG. 10, a curve-x (plotting with*) on a change ofabeam internal stress and capacitance between the both beams when apotential difference between the beams 81, 82 is V=0 in the case oftaking W1=4 μm and W2=2 μm on the S-formed beams 81, 82 having Δy=2 μm.By thus thickening the beam 81, the way of deformation due to residualstrain is different from that of the beam 82, resulting in a greatcapacitance change between the both. The extreme form, for enhancing therigidity of beam 81, is to make the beam 81 as a fixed electrode.However, in this case, the capacitance will change furthermore due to aninternal stress change.

[0077] There is, as another method for controlling the beams contactpoint, a method of providing a thickness t1 of the beam 81 greater thana thickness t2 of the beam 82, for example. There is shown, in FIG. 10,a curve-y (plotting with Δ) on a change of a beam internal stress andcapacitance between the both beams when the potential difference betweenthe beams 81, 82 is V=0 in the case of taking t1=4 μm and t2=2 μm.Unlike from the method to increase the width, thickness increaseapparently obtains an effect to keep capacitance nearly constant withoutundergoing the affection of internal stress.

[0078]FIG. 11 shows one example of a method for manufacturing a switchstructured as in the above. FIG. 11 is a sectional view along line A-A′in FIG. 8B, showing a state that an insulation film 91, sacrificiallayer 92 and photolithography-patterned resist 86 is formed on asubstrate 90, to form metal beams 81, 82, 83 between the patterned oneof photoresist 86 by electroplating. The seed layer 87 for a beam 81, 83is grounded. However, a seed layer 88 for a beam 82 is controlled by aswitch 89 such that it is grounded until a time T but is made equal toan anode potential V after the time T. The anode potential V is providedby an anode electrode 93. The use of such an electroplating processforms beams 81, 82, 83 as metal layers having the same height, beforethe time T. However, at time T and therafter, no plating is formed onthe beam 82. Thus, beams can be formed that are adjacent but differentin thickness.

[0079] In this manner, by merely providing a beam forming a variablecapacitance structure with a slight flexure, it is possible to suppressa characteristic change in pull-in voltage, capacitance or the like dueto residual stress or thermal expansion, as causing a problem in asmall-lined beam structure. Meanwhile, because the degree of flexure is,for example, approximately several μm for an electrode length L=500 μm,the resistance component of the beam itself is nearly the same as thatof a straight-lined beam. Also, there is no need to provide a flexurestructure besides the beam structure, and no prevention against deviceminiaturization. Furthermore, during a fabrication by a semiconductorthin-film process, flexure is determined by mask-rendering and henceeasy to form.

[0080] The switch using a flexure structure can be broadly diverted asvariable capacitive element to other devices. For example, in case thebeam is made as a mechanical resonator to use resonance of its lateralvibration and beam surface treatment so that a certain kind of gascomponent can be enhanced in absorbability to a beam surface, beam massvaries due to gas adsorption, to vary resonant frequency. Accordingly,this can be utilized as a gas concentration sensor. In this case, if itshould be structured by a resonator of a straight both-end-supportedbeam and adjacent fixed electrode, when the beam internal stress ischanged by the variation in beam residual stress resulting frommanufacture process or surrounding temperature change, problematicallythe resonant frequency greatly changes. However, such resonant frequencycan be moderated by using adjacent movable beams having a flexure formas in embodiment 4.

[0081] Using a parameter representative of a flexure form of the beamshown in FIG. 9, FIG. 12 shows a relationship between an internal stressand a primary resonant frequency. There is appeared a tendency similarto the feature of the relationship between an internal stress and apull-in voltage of FIG. 9. By increasing the curvature degree of S-form(Δy), resonant frequency change can be suppressed.

[0082] Incidentally, the foregoing embodiments explained the cases usingthe first, second and third of three beams, four beams or more can becomprised to structure a switch wherein three beams are for makingoperations according to the embodiments.

5. Fifth Exemplary Embodiment

[0083]FIG. 13A and 13B are side views showing a switch structureaccording to embodiment 5 of the invention. FIG. 13A is in a switch offstate while FIG. 13B is in a switch on state.

[0084] On a substrate 106, provided are a conductive pillar 108connected to an input terminal to input a signal and a conductive pillar109 connected to an output terminal to output a signal. Abeam-structured movable electrode 104 is suspended between the pillars108, 109. A fixed electrode 105 is arranged in an intermediate positionbetween the pillars 108 and 109 on the substrate 106. By applying anelectrostatic force between the movable electrode 104 and the fixedelectrode 105, the movable electrode 104 is moved toward the fixedelectrode 105. The movable electrode 104 is formed on a movable member103. The movable member 103 is structured by an ICPF (Ionic ConductingPolymer gel Film). The ICPF has an internal stress to vary dependingupon an application voltage, as shown in FIG. 14. By using this nature,the spring constant of the movable member 103 can be varied.

[0085] Now, switch operation is explained with reference to FIG. 15. InFIG. 15, the upper shows a position of the movable electrode 104 whilethe lower shows a change of spring constant in time of the movableelectrode 104. The neutral position the electrode 104 is not applied byan electrostatic force is assumably zero. When an electrostatic force iscaused between the movable electrode 104 and the electrode 105 tothereby attract the movable electrode 104 toward the electrode 105, acontrol voltage 107 is applied to the movable member 103 such that thespring constant of the movable member 103 assumes a minimum. At thistime, because the spring force is minimized, the movable member 103 andmovable electrode 104 is rapidly pulled in by an electrostatic forcewithout being interfered by the spring force.

[0086] Next, when the movable electrode 104 is detached from theelectrode 105, such a voltage as maximizing the ICPF spring force ispreviously applied to the movable member 103 by a control voltage 107,thus maximizing the spring force. By putting the electrostatic force offbetween the movable electrode 104 and the fixed electrode 105, themovable member 103 and movable electrode 104 rapidly returns to apredetermined position by the spring force.

[0087] Because polymeric gel generally has a response time ofapproximately several ms to a control signal, expanding/contracting apolymeric gel cannot be used as a drive force for a switch requiringhigh-speed response. There is a sufficient response time in changing thespring force of the movable member 103 into a state the switch is held.In this manner, high-speed response is made feasible by making thespring force of the movable member 103 to optimal values respectivelyupon pulling in and out.

[0088] The material used for the movable member 103 may be, besidesICPF, a material that the physical value is to vary depending uponexternal control, e.g. a polymeric gel or piezoelectric material for usein artificial muscle. Meanwhile, in case the movable member is formed ofa conductive material, the movable electrode 104 and the electrode 105can be formed in one body.

[0089] As in the above, the switch of the invention has an effect thatresponse time shortening and application voltage reduction can berealized by making three beams all movable. Furthermore, in caseadaptively selecting the number of using beams to provide an optimalimpedance in accordance with an application frequency, there is anadvantageous effect to realize a switch having no frequencycharacteristic. Meanwhile, the flexure structure of beams can suppressagainst switch characteristic change due to internal stress change.

What is claimed is:
 1. A switch comprising: first, second and thirdbeams arranged with constant spacing one from another; and voltageapplying means for providing a direct current potential to the first,second and third beams and applying a driving force to the first, secondand third beams.
 2. A switch according to claim 1, wherein, when turningon the switch, the voltage applying means generates a drive forcebetween the first beam and the second beam thereby placing the firstbeam and the second beam into a contact and electric coupling while,when turning off the switch, a drive force is caused between the secondbeam and the third beam thereby isolating between the first beam and thesecond beam.
 3. A switch according to claim 1, wherein the first, secondand third beams are arranged with a spacing satisfying a predeterminedisolation.
 4. A switch according to claim 2, wherein the voltageapplying means is to apply a direct current potential to the first,second and third beams for a predetermined time.
 5. A switch accordingto claim 2, wherein the third beam is moved after electric couplingbetween the first beam and the second beam.
 6. A switch according toclaim 1, wherein the beam is changed in a spring constant by changing ashape of the beam.
 7. A switch according to claim 1, wherein the secondbeam is made smaller in shape as compared to the first and third beams.8. A switch according to claim 2, wherein the second beam is madesmaller in shape as compared to the first and third beams, and, in astate the first and second beams are electrically coupled together, thethird beam is applied by a drive force from the first beam and the thirdbeam is moved toward the first and second beams.
 9. A switch accordingto claim 2, wherein, when the first and second beams are electricallycoupled together, the first beam and the second beam are switched inpotential each other, to cause a drive force on the third beam.
 10. Aswitch according to claim 2, wherein a drive force is not caused on thethird beam and the third beam is not moved.
 11. A switch according toclaim 1, wherein the first beam is connected to an antenna end, thesecond beam is connected to an input terminal and the third beam isterminated at a predetermined resistance value.
 12. A switch accordingto claim 2, when turning off the switch, said second beam and the thirdbeam are previously placed in contact. 13 A switch comprising a group ofswitches, said group of switches is formed by arranging a plurality ofswitches according to claim 1 in parallel.
 14. A switch according toclaim 1, wherein any of the first, second and third beams is formed of ametal.
 15. A switch according to claim 1, wherein the first, second andthird beams are arranged horizontally, any of the first, second andthird beams are to move horizontally.
 16. A switch according to claim 1,wherein the first, second and third beams are arranged vertically, anyof the first, second and third beams are to move vertically.
 17. Aswitch according to claim 11, wherein capacitances are arranged betweenthe first beam and the antenna end and between the second beam and theinput terminal.
 18. A switch according to claim 1, wherein the switch isto be operated in a vacuum or in an inert gas.
 19. A switch according toclaim 13, wherein said group of switches is formed by arranging aplurality of switches in the number symmetrically about the antenna end.20. A switch according to claim 1, wherein adjacent two or three of thefirst, second and third beams are bent in a same shape.
 21. A switchaccording to claim 20, wherein any of the first, second and third beamsis made in an S-form.
 22. A switch according to claim 20, wherein any ofthe first, second and third beams is different in thickness from anadjacent one thereof.
 23. A switch according to claim 1, wherein thedrive force is an electrostatic force.
 24. A switch according to claim1, wherein the switch is formed by a semiconductor process.
 25. A switchcomprising: an electrode arranged on a substrate; a movable electrode tocontact with the electrode and has as a constituent element a movablemember having an internal stress to vary depending upon a voltageapplied; first voltage applying means for causing an electrostatic forceat between the electrode and the movable electrode; and second voltageapplying means for applying a voltage to the movable member.
 26. Aswitch according to claim 25, wherein the movable member is structuredof a polymeric gel.
 27. A switch according to claim 25, wherein themovable electrode is structured by forming a conductive material on asurface of the movable member.
 28. A switch according to claim 25,wherein, when turning on the switch, the first voltage applying meansgenerates an electrostatic force between the movable electrode and theelectrode and the second voltage applying means applies a controlvoltage to the movable member such that a spring constant of the movablemember is minimized while, when turning off the switch, the secondvoltage applying means applies a control voltage to the movable membersuch that the spring constant of the movable member is maximized to putoff the electrostatic force due to the first voltage applying means. 29.A switch according to claim 25, wherein the switch is formed by asemiconductor process.